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Topic starter
November 27, 2024 3:42 pm
I have a mountain cabin with electrical power supplied by a utility company. The power drops out 2-3 times per year in the dead of winter due to snow and wind. It usually goes down for a day or two and then comes back up. I need to keep the electrically powered (gas) furnace blower on during power outages in order to keep water pipes from freezing. I do not have solar panels.
I've decided on LiFePO4 batteries to be used for a UPS system and just ordered an EGS Indoor Wall mount 48V 280aH battery along with a Victron 3kw inverter/charger.
I'm an electrical engineer but a newbie to UPS installations and how exactly battery BMSs work with inverters. I've read the user manuals for both the battery and inverter but many details are assumed or missing.
Here's my question: should I run the battery/inverter in open loop, or close loop configuration? My understanding is that in open loop, the inverter controls the battery charge rate and level, whereas in close loop, the battery controls the inverter and charge rate and level. If I run the system open loop, what does the battery BMS do since the inverter is in control of charge current, absorbtion, float voltage, etc?
The reason I am considering open loop mode is because then (I think) I can set the state of charge to say 90-95% in order to preserve battery life since these batteries will only discharge 2 or 3 times per year. My understanding is that if they are at 100% SOC for long periods, that puts a lot of stress on the cells and will shorten battery life. My understanding is that if I run close loop mode, the BMS will keep the batteries charged to 100%. Is this correct? While in open loop mode, I could keep an eye on cell balance and every once in a while, maybe once a month in the winter, charge them to 100% for cell balancing and then discharge them to 90% again. In the summer months I would discharge the battery to 50% and then disconnect it.
I'm also confused about what controls whether the system runs in open or close loop mode? If I establish CAN communication between the inverter and battery, does this automatically put the system in close loop mode, or does something else need to happen? Also, can I have CAN communication and still be running in open loop mode?
I'd be much obliged for any input that will educate me, make me less confused and suggest what configuration I should run for best system reliability and battery lifetime.
thanks!
Dave
November 27, 2024 4:00 pm
I am not familiar with the Victron systems. However, I am aware that with the EG$ inverters you can set the "charge to" and the "start to charge" levels based on SOC when in closed loop. I would think that the Victron would as well.?.
Not answering all of your questions but it's a start. 🙂
JB
Sometimes the easiest solution is the best solution.
Sometimes not.
November 27, 2024 9:18 pm
I recommend reviewing our white paper on closed-loop versus open-loop systems for a detailed explanation. Personally, I prefer the BMS communication provided by a closed-loop setup. Regarding leaving the CAN communication connected while operating in open loop, this is possible, but there would be no active communication between the two systems.
Topic starter
November 28, 2024 5:03 pm
@JB
Thanks for your response. I'm hoping Victron can do this.
Ok, looks like closed loop is definitely the way to go. Is there a document somewhere that describes what exactly happens in closed loop mode?
In particular:
- What controls the battery SOC, is it the BMS, or is it the inverter?
- What I really want to do is just limit the SOC to ~95% unless I want to re-balance the cells, then I'd bump it up 100%. Assuming the EG4 SOC is best determined and regulated by the BMS, can the SOC limit be set somehow? Is this something that can typically be set over the CAN bus via the inverter interface? I'm using a Victron inverter. Is there a battery command list somewhere? I don't see one in the manual.
thanks - Dave
November 28, 2024 7:54 pm
@davede Could you share the model number of the Victron inverter. If I get some time I can poke around in the manual and see what I can.
In regard to your particulars:
- What controls the battery SOC, is it the BMS, or is it the inverter? My understanding (I am NOT a BMS expert and hopefully someone that is will correct me when I misspeak) is that in closed loop, the BMS will report the SOC to the inverter and to some extent control current to and from the battery to help provide the best charge and balancing scenario. I also get the impression that some inverters will take the SOC and VDC readings from the BMS and use that information to control the charge cycles. I get the unconfirmed impression that this is typically the case in closed loop but I am not certain.
Thanks,
JB
Sometimes the easiest solution is the best solution.
Sometimes not.
Topic starter
November 28, 2024 8:29 pm
@JB
The inverter is a Victron Multiplus II, 48V, 3KVA unit.
https://www.currentconnected.com/product/victron-48v-multiplus-2-3000va-ul1741/
I'm also going to use a Cerbo GX and a GX touch 50 display
https://www.currentconnected.com/product/cerbo-s-gx-lite-system-controller/
https://www.currentconnected.com/product/gx-touch-display-cerbo-gx/
In the ideal world, I would want to be able to program the EG4 battery BMS to limit the battery SOC to whatever I want, say 90% or 100%. In addition, configure the system for closed loop mode such that the BMS controls and limits the inverter charging output (or the inverter reads the SOC level from the BMS and stops charging when the programmed level is reached). I would also want to be able the read the battery cell voltages, SOC, etc.
I also have no idea what actually puts the system in closed loop mode. It's one thing to connect the CAN bus and establish communication between the two so that battery parameters can be read by the inverter, but what actually makes the system run in closed loop mode whereby the BMS controls the inverter or the inverter controls the BMS, and again, who's in charge, the BMS or the inverter?
thanks! - Dave
November 29, 2024 3:06 pm
@davede as far as having the BMS completely control the charge cycle, I am just not certain. I am not extremely familiar with the detailed functionality of BMS's. It does appear that the Victron equipment will (or at least can) set charge limits by SOC - please see --> 4. Configuration
Regarding reading battery cell voltages etc. I don't know what level of compatibility the EG4 wallmount batteries BMS has with the Victron system you are looking at.
If you are looking at purchasing from Current Connected, I would suggest reaching out to their pre-sales team. I have worked with them, and they are very good at what they do.
I would suggest telling them what you want to accomplish and what the planned set up is.
1 - Grid tied - no solar - Planned as UPS only and only used on rare occasions.
2 - Since you plan that it will only be used on rare grid outage occasions you would like to be able to do the following:
a - Maintain the batteries at XX% charge at all times.
b - You want to monitor the cell voltages, SOC, etc. via the monitoring system
c - Can this all be done with the system you have spec'd
d - Will they be able to aid in configuring the system to meet your needs.
As you visit with them drop in a few questions as to the specifics of how the BMS work and communicates with the Victron.
Sorry I wasn't much help, but BMS is just not my forte wish I understood them better and someday I will. 🙂
Hopefully one the BMS gurus will chime in with better answers.
Best of luck and please keep us posted.
JB
Sometimes the easiest solution is the best solution.
Sometimes not.
